1. Field of the Invention
The invention relates to a sputtering apparatus, and particularly to a sputtering apparatus which is used in a film depositing step for manufacturing a semiconductor integrated circuit or the like.
2. Description of the Related Art
In a thin film deposition using sputtering, and particularly in sputtering used in a film depositing step for manufacturing a highly-integrated semiconductor device, it is strongly requested to deposit a film at the bottom of a fine hole of a high aspect ratio with an excellent step coverage performance, that is, to improve the bottom coverage ratio.
In order to comply with the requirement, improvements have been made so that a film is deposited while allowing only sputter particles of a small incident angle to enter a fine hole. One of the improvements is a technique called collimate sputtering.
FIG. 10 is a view schematically illustrating a collimate sputtering apparatus as an example of a conventional sputtering apparatus in which the bottom coverage ratio is improved. In the apparatus shown in FIG. 10, a cathode 2 and a substrate holder 3 are disposed so as to oppose each other in a vacuum vessel 1. The cathode 2 comprises a magnet mechanism 4, and a target 5 which is located in front of the magnet mechanism 4. A substrate 30 on which a film is to be deposited is placed on the front face of the substrate holder 3.
A collimator 6 is disposed in a space between the cathode 2 and the substrate holder 3. The collimator 6 has a structure in which a number of small cylindrical members are arranged in a segmental form so that their height directions coincide with a direction perpendicular to the substrate 30 (hereinafter, the direction is referred to as the axial direction), whereby many flow paths for sputter particles are segmentally formed along the axial direction. This structure is often called a "grid-shaped" or "honeycomb" structure.
Sputter particles emitted from the target 5 are distributed in accordance with the cosine law. Therefore, also many sputter particles of a large incident angle enter the collimator 6. However, most of such sputter particles are deposited on the wall faces of the flow paths of the collimator 6, with the result that sputter particles emitted from the collimator 6 mainly consist of those of a small incident angle. Consequently, only sputter particles of a small incident angle impinge on the substrate 30, so that the step coverage performance for the bottom of a fine hole formed in the surface of the substrate 30 is improved.
In the collimate sputtering apparatus described above, however, the deposition of sputter particles on the collimator 6 reduces the sectional areas of the flow paths of the collimator 6, with the result that the amount of sputter particles which can pass through the collimator 6 is reduced with the passage of time. Therefore, the sputtering rate is gradually lowered.
Recently, an apparatus which is called a low-pressure long-distance sputtering apparatus and in which the distance between a target and a substrate (hereinafter, the distance is referred to as the TS distance) is increased (3 to 6 times that of the conventional apparatus) has been developed as a sputtering apparatus which is free from the above problem and which has a high bottom coverage ratio. FIG. 11 is a view schematically illustrating a low-pressure long-distance sputtering apparatus as another example of a conventional sputtering apparatus.
In the apparatus shown in FIG. 11, in the same manner as that of FIG. 10, a cathode 2 and a substrate holder 3 are disposed so as to oppose each other in a vacuum vessel 1, a target 5 is located in front of a magnet mechanism 4, and a substrate 30 is placed on the front face of the substrate holder 3. The TS distance is set to be, for example, about 150 to 360 mm. The pressure of the interior of the vacuum vessel 1 is set to be lower than that in the conventional system or to be about 1 mTorr or less. This pressure reduction is conducted in order that the mean free path of sputter particles is increased and sputter particles are less scattered. Since scattering of sputter particles is reduced in level, many sputter particles can impinge on the substrate in a direction substantially perpendicular to the substrate, thereby enabling the bottom coverage ratio of a fine hole to be improved.
Specifically, for example, Japanese Patent Unexamined Publication No. Hei. 7-292474 describes that the bottom coverage ratio can be improved under the conditions that the diameter of a target is 250 mm, the diameter of a substrate is 200 mm, the TS distance is 300 mm, and the pressure is 3.times.10.sup.-2 Pa.
As shown in Table 3 of the publication, however, the film deposition rate is largely lowered when the TS distance is increased in order to improve the bottom coverage ratio. Consequently, although effective in film deposition in a fine hole in a process for 256 Mbits or more (line width: 0.25 .mu.m, aspect ratio: 4 to 6), the technique of low-pressure long-distance sputtering remains to have a problem in productivity. When the TS distance is reduced in order to increase the film deposition rate, the bottom coverage ratio is lowered, and hence it is difficult to apply the technique to a process for 256 Mbits or more. In other words, in low-pressure long-distance sputtering, the film deposition rate and the bottom coverage ratio are in tradeoff relationship and not compatible with each other.
On the other hand, a sputtering process is further required to adapt to a substrate of an enlarged size. In a process of manufacturing a semiconductor device such as that described above, the size of a substrate tends to be enlarged in order to manufacture a larger number of devices from one substrate and improve the productivity. Also in a sputtering process to be conducted on a glass substrate in manufacture of a liquid crystal display device, the size of a substrate tends to be enlarged in order to widen the display area.
Such an enlarged size of a substrate is complicatedly entangled with the factors of the TS distance and the film deposition rate in low-pressure long-distance sputtering described above.
First, an enlarged size of a substrate raises a problem in that, also in low-pressure long-distance sputtering, the bottom coverage ratio is insufficient in a portion remote from the center or a peripheral portion of the substrate. This problem will be described with reference to FIGS. 12, 13 (A) and 13 (B).
FIGS. 12, 13 (A) and 13 (B) illustrate the problem which arises in the film deposition on a large substrate with using the apparatus of FIG. 11. FIG. 12 is a partial diagrammatic view of the target and a substrate in the apparatus, and FIGS. 13 (A) and 13 (B) are section views showing the bottom coverage ratios of the vicinity of the center of the substrate and the peripheral portion.
As shown in FIG. 12, the target 5 and the substrate 30 are disposed in parallel so as to oppose each other, and their center axes 20 (the axes passing the center and perpendicular to the surface) are on the same line. FIG. 12 shows only portions which are on the one side with respect to the center axes 20.
When sputtering is done, erosion occurs in the surface of the target 5 as indicated by the hatched portion in FIG. 12. In fine holes 301 which are respectively formed in the vicinity of the center of the substrate 30 and the peripheral portion, a film is deposited in different manners as shown in FIGS. 13 (A) and 13 (B). In the vicinity of the center of the substrate 30, as shown in FIG. 13 (A), a film 302 is deposited at the bottom of the fine hole 301 with an excellent step coverage performance. In contrast, in the peripheral portion of the substrate 30 which is outside the portion of the same diameter as that of the target 5, the number of sputter particles which impinge from the side of the center axis at a large incident angle is increased, and hence the fine hole 301 has a state in which, as shown in FIG. 13 (B), the film 302 is deposited on the wall face on the side of the peripheral edge of the substrate 30 but is not deposited on the wall face on the side of the center axis and the bottom face.
In the case of deposition of a barrier metal on the inner face of a contact hole, such a state results in a fatal defect. When a large substrate is to be used, therefore, the size of the target must be correspondingly increased.
This problem of the increased size of a substrate is complicatedly entangled with the above-mentioned problem of incompatibility of the film deposition rate and the TS distance required for the bottom coverage ratio, whereby the problems are worsened. This will be described with using data in the company of the assignee of the present application.
FIGS. 14 to 17 show experimental data relating to low-pressure long-distance sputtering. FIG. 14 shows data indicating the dependence of the bottom coverage ratio on the pressure and the TS distance, and FIG. 15 shows data indicating the dependence of the distribution of the sheet resistivity of an obtained thin film on the pressure and the TS distance. FIGS. 16 and 17 show data indicating relationships between the bottom coverage ratio and the aspect ratio. FIG. 16 shows the case where the TS distance is 340 mm, and FIG. 17 that where the TS distance is 260 mm. These data are obtained under the conditions that the diameter of the substrate is 6 inches and that of the target is 269 mm.
As shown in FIG. 14, it can be seen that the bottom coverage ratio is improved in a low pressure range. The bottom coverage ratio in the case where the TS distance is 100 mm is higher than that in the case where the TS distance is 65 mm. Furthermore, the bottom coverage ratio in the peripheral portion of the substrate is higher than that in the vicinity of the center of the substrate.
As shown in FIG. 15, when the TS distance is increased, the uniformity of the distribution of the sheet resistivity tends to be impaired. However, this tendency is moderated by reducing the pressure. Specifically, in the case where the pressure is 2.0 mTorr or less, the distribution of the sheet resistivity hardly changes even when the TS distance is increased.
Next, the relationships between the bottom coverage ratio and the aspect ratio will be discussed. As shown in FIG. 16, when the aspect ratio is 2, it is possible to attain a bottom coverage ratio of 40 to 45%. It is generally known that, when the bottom coverage ratio is about 15%, there arises no problem in properties of a device. Also from this point of view, it will be seen that the low-pressure long-distance sputtering method is a very excellent technique. In a collimate sputtering apparatus such as that shown in FIG. 10, the bottom coverage ratio is about 15%. From this, excellence of the low-pressure long-distance sputtering method will be again noted.
In FIG. 16, the mark 0 indicates the bottom coverage ratio in the vicinity of the center of the substrate, and the mark .oval-solid. that in the peripheral portion. In both the sets of data, the data are arranged on the same line. This shows that the bottom coverage ratio in the face of the substrate maintains high uniformity. The film deposition rate is similar to that of the collimate sputtering method or about 600 angstroms per minute. Namely, the film deposition rate is reduced to about 1/3 to 1/4 of that of sputtering of the conventional system.
In contrast, when the TS distance is reduced to 260 mm, the film deposition rate is improved to 1,000 angstroms per minute. As shown in FIG. 17, however, the bottom coverage ratio is lowered to about 28 to 35% in the case of the aspect ratio of 2. Even in this case, the bottom coverage ratio is higher than that of the collimate sputtering method or 15%.
From these results, the bottom coverage ratio and the film deposition rate in the case where the substrate is increased in size or has a diameter of, for example, 300 mm will be studied. FIG. 18 is a view showing results of studies on the effects of the enlarged size of the substrate on the bottom coverage ratio and the film deposition rate. In FIG. 18, the hatched portions indicate the sectional shape of erosion in the target 5.
First, as shown in FIG. 17, an excellent bottom coverage ratio can be attained under the conditions that the diameter of the target is 269 mm and the TS distance is 340 mm (FIG. 18 (a)). This is applicable also to the case where the substrate 30 is smaller than the target 5 or has a diameter of 8 inches.
When the substrate 30 is made larger than the target 5 or has a diameter of 300 mm, also the target 5 must be enlarged to a size similar to that of the substrate 30 as described above. In this case, it is considered that, in order to attain a bottom coverage ratio of a similar level, the TS distance must be further increased.
This will be described by using the flying path of sputter particles from the deepest erosion portion as the representative. In many sputtering processes, in the region on a target where erosion occurs (hereinafter, referred to as the erosion region), a specific portion in a radial direction of the target tends to be eroded most deeply (hereinafter, such a portion is referred to as the deepest erosion portion), and sputter particles emitted from this portion dominantly affect the state of the film deposition. In planar magnetron sputtering or the like which is mainly used today, the erosion region forms a circumferential shape. In many cases, therefore, the deepest erosion portion has a circumferential shape.
FIGS. 19 and 20 are views illustrating the circumferential shape of the deepest erosion portion. FIG. 19 is a schematic perspective view of a magnet mechanism used in a conventional apparatus, and FIG. 20 is a schematic perspective view of a cathode used in the conventional apparatus. In an apparatus such as that shown FIGS. 10 or 11, the magnet mechanism 4 disposed in the back of the flat target 5 comprises a column-shaped center magnet 412 fixed onto a disk-shaped yoke 411, and a cylindrical peripheral magnet 413 which surrounds the center magnet 412 with leaving a gap therebetween.
The different magnetic poles appear on the front faces of the center magnet 412 and the peripheral magnet 413, respectively. For example, the center magnet 412 constitutes the S-pole and the peripheral magnet 413 constitutes the N-pole. In this case, lines of magnetic force emerging from the peripheral magnet 413 pass through the target 5 to leak from a certain portion of the surface of the target 5, arcuately warp as shown in FIGS. 19 and 20, enter another portion of the surface of the target 5, and then pass through the target 5 to reach the center magnet 412. These leakage lines of magnetic force are ranged along the shape of the gap between the center magnet 412 and the peripheral magnet 413, thereby forming a circumferential magnetic field as shown in FIGS. 19 and 20.
In a sputtering process using a function of a magnetic field, such as magnetron sputtering, electrons are captured by the magnetic field, so that the efficiency of ionizing gas molecules is improved. Consequently, the region of the target 5 which is sputtered by ions, i.e., the erosion region 50 has a shape corresponding to that of the magnetic field. In the above example in which a circumferential magnetic field is set, the region has a circumferential shape.
In magnetron sputtering, electrons perform the magnetron motion in a region where the electric field orthogonally crosses the magnetic field, and the ionization efficiency is maximum in the region. In the configuration of FIGS. 19 and 20, therefore, the orthogonal relationship between the electric field and the magnetic field is established in the summit portion of the arcuate leakage lines of magnetic force, and strong erosion tends to occur in the portion below the summit portion. In other words, the deepest erosion portion draws a circumferential shape located below the summit portion of the arcuate leakage lines of magnetic force.
As described above, sputter particles are vigorously emitted from the deepest erosion portion. Consequently, it seems that the geometrical arrangement of the deepest erosion portion most significantly affects the state of the film deposition on the substrate. It is considered that, when the target 5 and the substrate 30 are coaxially arranged so as to oppose each other as shown in FIGS. 10 and 11, the deepest erosion portion of the half circumferential portion or one half of the target 5 affects the film deposition in the corresponding half region of the substrate 30 but does not affect that in the other half region, because the surface of the other half region of the substrate 30 is affected by erosion in the other half circumferential portion of the target 5.
Sputter particles emitted from the deepest erosion portion of one half circumferential portion will be considered here. Among such sputter particles, those impinging on the vicinity of the center of the substrate 30 have the largest incident angle with respect to the substrate 30. In the case where the radius of the deepest erosion portion is not larger than one half of the radius of the target 5, sputter particles impinging on the peripheral portion of the substrate 30 have the smallest incident angle. However, such a case seldom happens.
In the example of FIG. 18 (a) in which the target has a diameter of 269 mm, when the deepest erosion portion is produced at a position separated from the center axis 20 by, for example, 70 mm (the diameter: 140 mm), the incident angle .theta. of sputter particles impinging on the vicinity of the center of the substrate 30 is about 11.6.degree. under the conditions that the TS distance is 340 mm.
In contrast, when the substrate 30 of a larger size or having a diameter of 300 mm is used, also the target 5 must be enlarged so as to have a similar size as described above. As shown in FIG. 18 (b), when the target 5 which is slightly larger than the substrate 30 or has a diameter of 314 mm is used, and the deepest erosion portion is produced at a position corresponding to a diameter of 163 mm, if the same TS distance as the above is employed, the incident angle .theta. of sputter particles impinging on the vicinity of the center is increased to about 13.5.degree.. In order to set the same incident angle as that of FIG. 18 (a) and attain a similar bottom coverage ratio, therefore, the TS distance must be increased to a large value or 397 mm. When the TS distance is increased to such a large value, the film deposition rate is impaired to a low level at which the film deposition is practically impossible.
To comply with this, for example, as shown in FIG. 18 (d), the TS distance is set to be 303 mm (similar to the diameter of the substrate) in a practical range, so that the incident angle .theta. in the vicinity of the center is about 15.0.degree.. Namely, the incident angle is increased to be (15.0/11.3)=1.3 times that of FIG. 18 (b). The incident angle of 15.0.degree. is equal to that attained in the case where the target 5 of a conventional size or 269 mm (the diameter of the deepest erosion portion is 140 mm) is used and the TS distance is about 260 mm (FIG. 18 (c)). This configuration is identical with the sputtering from which the data of FIG. 17 were obtained, and can obtain only a bottom coverage ratio of about 28 to 35% for a fine hole of an aspect ratio of 2.
As described above, even when the low-pressure long-distance sputtering method is employed, it is difficult for the conventional configuration to improve the bottom coverage ratio while maintaining a required film deposition rate so as to comply with the tendency to enlarge a substrate.